1. Field of the Invention
The present invention relates to an image forming apparatus, for example a copying machine, a printer, a facsimile apparatus or the like, which is provided with a function of forming an image on a recording medium such as a sheet.
2. Description of the Related Art
Conventionally, as an image forming apparatus of this type, many digital image forming apparatus use an IAE method (image area exposure, FIG. 12) for subjecting an image part of an image to laser exposure. Exposure is performed by the IAE method because, with the IAE method, a line width can be made larger by increasing an amount of light and a lifetime of a laser can be longer due to a short laser irradiation time.
In addition, a UFS (Under-Filled Scanner, FIGS. 9 and 10) method having a polygon mirror surface wider than a width in the main scanning direction of light incident on the polygon mirror (an incident beam width) is used in a laser scan optical system. Since a width of a laser beam reflected on a polygon mirror is fixed in the UFS method, it has an advantage in that an amount of laser light is equal in the longitudinal direction of a photosensitive member.
Therefore, a conventional image forming apparatus often performs laser exposure of the IAE method using a polygon mirror of the UFS method.
However, as processing of an image forming apparatus is getting faster, rise in temperature and noises have emerged as problems because a large polygon mirror is used in the UFS method.
As a measure to cope with these problems, there is an OFS (Over-Filled scanner, FIGS. 7 and 8) method using a small polygon mirror.
In an OFS method a polygon mirror surface is narrower than an incident beam width and is smaller than in the UFS method. Thus, an OFS is excellent in speed and definition because heating and noises can be restrained, start-up is fast and a spot diameter of a laser beam can be made small.
Therefore, exposure is performed in a recent high-speed digital image forming apparatus by the IAE method using an OFS.
However, in the case of the above-mentioned conventional art, there are problems mentioned below.
Since in the OFS method a photosensitive member is irradiated with a part of a beam as shown in FIGS. 7 and 8, a reflected beam width varies in response to an angle of a laser incident on a polygon mirror. As a result, a amount of laser light is unequal in the longitudinal direction.
More specifically, there is a disadvantage in the OFS method in that an amount of laser light decreases from about 5 to 10% on an end side compared with a central part while there is hardly any decrease in a amount of laser light at an end in the longitudinal direction on a photosensitive member compared with a central part in the UFS method as shown in FIG. 4.
As a measure to cope with this disadvantage, there is a method of controlling a amount of laser light such that the amount of laser light is decreased when a central part in the longitudinal direction of a photosensitive member is irradiated and is increased when an end part is irradiated. There is also a method of decreasing a light amount at a central part by applying coating on a lens or a mirror on a path which a beam scanned by a polygon mirror reaches a photosensitive member.
However, controlling the amount of laser light where accuracy is required, a method of increasing accuracy in laser irradiation, or a method using a lens of a special shape is costly. A method of applying coating to a lens or a mirror is also costly and, in addition, loses a large amount of light, which may lead to shortage of the amount of laser light.
In addition, a potential unevenness is generated in a pitch shape in a drum""s circumferential direction because of an unevenness of a rotational period of a polygon, a rotational period of a driving gear of a photosensitive member, or the like in a laser exposed part of an electrostatic image created by a laser beam that is scanned using a polygon mirror (FIG. 11).
That is, the IAE method for making the laser exposed part an image part has a disadvantage in that an unevenness of a pitch shape, a white line or a black line is generated in an image part.
As a measure to cope with this disadvantage, there is a method of decreasing a plane tilt or the like of a polygon mirror to increase the accuracy of laser irradiation on a photosensitive member. There is also a method of using a flywheel for preventing a periodical unevenness of a photosensitive member, changing a material of a driving gear, or changing engagement of a driving gear. In addition, there is a method of making an unevenness less noticeable by increasing a development contrast potential (VCONT) and applying a lot of toner.
However, the method of increasing the development contrast potential VCONT has problems such as splash, scattering of toner and increased consumption of toner because a toner density is too high due to the increased amount of toner applied.
The present invention has been devised in order to solve the above-mentioned problems of the conventional art, and it is an object of the present invention to provide a high quality image forming apparatus that has no unevenness and less possibility of occurrence of a fog or a change in density and is capable of outputting a quality image at a high speed.
In the present invention, an image forming apparatus is provided, which comprises an image bearing member for bearing an electrostatic latent image; and exposing means for exposing said image bearing member to form an electrostatic latent image, wherein the exposing means has a rotational polygon mirror having an incident beam reflecting surface that is narrower than an incident beam width and exposes a non-image part of an image to form an electrostatic latent image.
According to the present invention, the exposing means is operated by an over-filled scanner method for exposure by a rotational polygon mirror for reflecting light on a surface narrower than an incident beam width and forms an electrostatic latent image by back area exposure for exposing a non-image part. Therefore, a high resolution image can be outputted at a high speed and prevention of occurrence of a fog or a scanning unevenness can be realized with reduced costs.
In addition, it is preferable that the image forming apparatus further comprises potential detecting means for detecting a surface potential of the image bearing member, development bias applying means for applying a development bias voltage to developing means for developing the electrostatic latent image; and image forming conditions operating means for calculating a DC component value of the development bias voltage applied by the development bias applying means in response to detection results of the potential detecting means.
In this way, since the image forming apparatus is provided with the image forming conditions operating means for controlling the DC component value of a development bias applied by the development bias power source in response to detection results of the potential detecting means, occurrence of a fog and a scanning unevenness can be prevented.
In addition, it is preferable that the image forming conditions operating means calculates the DC component value of the development bias voltage as a value found by adding a predetermined value to a potential in an exposed part that is exposed by the exposing means among surface potentials of the image bearing member detected by the potential detecting means.
In addition, it is preferable that the potential detecting means is arranged such that it detects a potential of an end in the longitudinal direction of the image bearing member, and the image forming conditions operating means calculates the DC component value of the development bias voltage as a value found by adding a predetermined value to a potential of an exposed part at the end in the longitudinal direction of the image bearing member detected by the potential detecting means.
In addition, it is preferable that the potential detecting means is arranged such that it detects a potential of a region other than the end in the longitudinal direction of the image bearing member, and the image forming conditions operating means estimates a potential at an end in the longitudinal direction of the image bearing member from the surface potential of the image bearing member detected by the potential detecting means based on a characteristic of an amount of light irradiated by the exposing means and calculates the DC component value of the development bias voltage as a value found by adding a predetermined value to the estimated potential at the end.
In addition, it is preferable that the potential detecting means is arranged such that it detects a potential of a region other than the end in the longitudinal direction and the central part of the image bearing member.
In addition, it is preferable that, if the image forming conditions operating means determines that an absolute value of a potential of the exposed part of the image bearing member is larger than a predetermined potential, the image forming conditions operating means controls an amount of light on the surface of the image bearing body irradiated by the exposing means such that the detected potential of the exposed part becomes the predetermined value.
In addition, it is preferable that, if the image forming conditions operating means determines that an absolute value of the estimated potential of the end in the longitudinal direction of the image bearing member is larger than a predetermined potential, the image forming conditions operating means controls an amount of light on the surface of the image bearing member irradiated by the exposing member such that the potential at the end becomes the predetermined potential.
In addition, it is preferable that the predetermined value is a value for preventing occurrence of a fog in the exposed part.
In addition, it is preferable that the predetermined value is a value for preventing occurrence of a fog at the end in the longitudinal direction of the image bearing member.
In addition, it is preferable that the image forming conditions operating means controls an amount of light on the surface of the image bearing member irradiated by the exposing means based on a difference between the DC component value of the development bias voltage and the potential of the end in the longitudinal direction of the image bearing member.
In addition, it is preferable that the image bearing member is an amorphous silicon (a-Si) photosensitive member.
As described above, influences on an image density by endurance can be almost eliminated and a quality image close to an initial state can be realized by using the a-Si photosensitive member.
In addition, in the present invention, a method of controlling an image forming conditions of an image forming apparatus for back-area exposing an image bearing member by light scanned by the over-filled scanner method to form an electrostatic latent image is provided, which comprises: detecting a surface potential of an image bearing member for bearing an electrostatic latent image; and calculating a DC component value of a development bias voltage applied to a developing means for developing the electrostatic latent image on the image bearing member.
In addition, it is preferable that the surface potential of the image bearing member to be detected is a potential of an exposed part at an end in the longitudinal direction of the image bearing member, and the DC component value of the development bias voltage is calculated as a value found by adding a predetermined value to the detected potential of the exposed part when the DC component value of the development bias voltage is calculated in response to the detected surface potential of the image bearing member.
In addition, it is preferable that the surface potential of the image bearing member to be detected is a potential of a region other than the end in the longitudinal direction of the image bearing member, and the potential of the end in the longitudinal direction of the image bearing member is estimated from the detected surface potential of the image bearing member based on a characteristic of the amount of exposing light irradiated on the image bearing body, and the DC component value of the development bias voltage is calculated as a value found by adding a predetermined value to the estimated potential of the end in the longitudinal direction the image bearing member when the DC component value of the development bias voltage is calculated in response to the detected surface potential of the image bearing member.
In addition, it is preferable that it is determined whether or not an absolute value of the detected potential of the exposed part of the end in the longitudinal direction of the image bearing member is larger than a predetermined potential, and if the absolute value of the detected potential of the exposed part of the end in the longitudinal direction of the image bearing member is larger than the predetermined potential, an amount of light irradiated on the surface of the image bearing member is controlled such that the potential of the exposed part becomes the predetermined value.
In addition, it is preferable that it is determined whether or not an absolute value of the estimated potential of the end in the longitudinal direction of the image bearing member is larger than a predetermined value, and if the absolute value of the estimated potential of the end in the longitudinal direction of the image bearing member is larger than the predetermined potential, an amount of light irradiated on the surface of the image bearing member is controlled such that the potential at the end becomes the predetermined value.